Fix compilation under chromium checkouts.
Fix compilation under chromium checkouts.

This is a fix to hide pdfium's safe_conversions.h from the
higher-level callers.

R=thestig@chromium.org

Review URL: https://codereview.chromium.org/1132163002

andoma/pdfium,chenjian-rits/HelloWorld,chenjian-rits/HelloWorld,wwxxyx/pdfium0,azunite/libpdfium,rouault/pdfium,DrAlexx/pdfium,bradh/pdfium,was4444/pdfium,klokantech/pdfium,nathanstitt/pdfium,azunite/pdf_clone,bradh/pdfium,DrAlexx/pdfium,nathanstitt/pdfium,was4444/pdfium,klokantech/pdfium,azunite/pdf_clone,andoma/pdfium,DrAlexx/pdfium,pwaller/pdfium,was4444/pdfium,wwxxyx/pdfium0,pwaller/pdfium,azunite/pdf_clone,azunite/libpdfium,klokantech/pdfium,andoma/pdfium,nathanstitt/pdfium,azunite/libpdfium,chenjian-rits/HelloWorld,rouault/pdfium,andoma/pdfium,azunite/libpdfium,wwxxyx/pdfium0,DrAlexx/pdfium,bradh/pdfium,rouault/pdfium,was4444/pdfium,pwaller/pdfium